Doing this is one of my earliest memories. I think I was 5 or 6. I hadn't even mastered reading yet and I was typing a game into an IBM XT clone (think it was QBasic) one character at a time. Don't think I would have bothered with a tax prep program, though. Now that's dedication.

Edit: Could not have been QBasic as it wasn't released until 1991 and I was doing this in 1987-1988. So maybe GW Basic? Whatever came standard on an IBM XT clone from Taiwan.